class Adder
  def add( a, b )
    a.to_f + b.to_f
  end
end

class Subtractor
  def subtract( a, b )
    a.to_f - b.to_f
  end
end

class Multiplier
  def multiply( a, b )
    a.to_f * b.to_f
  end
end

class Divider
  def divide( a, b )
    a.to_f / b.to_f
  end
end

class Calculator
  attr_writer :adder
  attr_writer :subtractor
  attr_writer :multiplier
  attr_writer :divider
  attr_writer :functions

  attr_accessor :memory

  def add( a, b=memory )
    @adder.add( a, b )
  end

  def subtract( a, b=memory )
    @subtractor.subtract( a, b )
  end

  def multiply( a, b=memory )
    @multiplier.multiply( a, b )
  end

  def divide( a, b=memory )
    @divider.divide( a, b )
  end

  def function( name, *arguments )
    @functions[ name ].compute( *arguments )
  end
end